1. Field of the Invention
The present invention relates to magnetic resonance imaging for imaging blood vessels and tissue of a subject (patient) on the basis of a magnetic resonance phenomenon occurring in the subject. More particularly, this invention is concerned with a magnetic resonance imaging (MRI) system and magnetic resonance (MR) imaging method that provides tissue/blood contrast images of a higher quality.
In one aspect, such images are provided by utilizing MT (magnetization transfer) pulses that are able to greatly raise contrast between blood (or flow of blood) and tissue. In another aspect, such high-quality tissue/blood contrast images are obtained by acquiring a plurality of echo signals responding to one exciting pulse incorporated in a pulse sequence to which both a degree of resolution and a time for echo acquisition are optimized. In this later case, a patient's breath hold is improved by using an easier self-navigation technique.
2. Description of the Related Art
Magnetic resonance imaging (MRI) is a technique for magnetically exciting nuclear spins of a subject positioned in a static magnetic field by applying a radio-frequency (RF) signal with the Larmor frequency, and reconstructing an image using MR signals induced by the excitation.
One field of magnetic resonance (MR) imaging is MR angiography. A phase contrast method is one technique for MR angiography, which uses pulses referred to as flow encoding pulses. Another method for MR angiography is to utilize MT effects (or what may be referred to as MTC (magnetization transfer contrast effects) to produce images to produce contrast between blood (flow of blood) and tissue. Recently this technique has frequently been used. One such example is disclosed by U.S. Pat. No. 5,050,609 (Magnetization Transfer Contrast and Proton Relaxation and Use thereof in Magnetic Resonance Imaging).
The research of MT effects originates from the study of a ST (saturation transfer) method by Forsen & Hoffman (refer to “Forsen et al., Journal of Chemical Physics, Vol.39(11), pp.2892-2901 (1963)”). The MT effects are based on chemical exchanges and/or cross relaxation between protons of a plurality of types of nuclear pools, such as free water and macromolecules.
Several conventional MR angiography techniques that use MT effects are discussed below.
In each of FIGS. 1A to 1C, the left-side graph represents the frequency spectra of free water and macromolecules, while the right-side one illustrates the exchange and relaxation relation of their magnetizations Mr and Mf. As shown in the spectra of protons of free water and macromolecules, the free water of which T2 (spin-spin) relaxation time is longer (T2 is approx. 100 msec) and the macromolecules of which T2 relaxation time is shorter (T2 is approx. 0.1 to 0.2 msec) resonate in the same frequency range. Since the T2 relaxation time of a free water signal is longer, its Fourier-transformed signal has a peak with a relatively narrow half-width, as shown. However, in the case of the signal of protons whose movement is restricted among macromolecules, such as protein, its Fourier-transformed signal exhibits a relatively broader half-width value, due to a shorter T2 relaxation time, no longer appearing as a distinct peak in the spectra.
When taking the resonance peak frequency fo of free water as the center frequency, a frequency-selective pulse serving as an MT pulse is applied to excite a frequency range shifted, for example, by 500 Hz from the center frequency fo of free water (that is, off-resonance excitation), as shown in the left side of FIG. 1B. This excitation causes the magnetization Hf of free water and those Hr of macromolecules, both of which are in equilibrium as shown in the right side of FIG. 1A, to change relative to one another as the magnetization Hf of free water moves to the Hr of macromolecules as shown in the right side of FIG. 1B. As a result of it, as illustrated in the left-side graph of FIG. 1C, the signal value of protons of free water decreases. Differences in signal values are caused between one region in which the chemical exchanges and/or cross relaxation between free water and macromolecules are reflected and the other region in which such chemical exchanges and/or cross relaxation is not reflected. These differences lead to differences in contrast between flow of blood and tissue, providing blood flow images.
At present, MR angiography based on MT effects is classified into spatially non-selective imaging and slice-selective imaging.
As an example of the former, “G. P. Pike, MRM 25, 327-379, 1992” describes a frequency-selective binomial pulse used as the MT pulse and applied in a spatially non-selective manner. Contrast between parenchyma and flow of blood is obtained according to a relation of “MT effects of parenchyma >MT effects of flow of blood.”
As an example of the latter imaging, a proposal by “M. Miyazaki, MRM 32, 52-59, 1994.” This paper teaches that a slice-selective MT pulse uses an RF excitation pulse having an application time that is relatively long and accompanied by gradient spoiler pulses. Application of such an MT pulse causes MR signals to be emanated from stationary parenchyma in a slice to be imaged to be lowered more than in a flow of blood that passes therethrough, due to its MT effects. MT effects received by flow of blood that comes into the slice also cause a signal decease (but, the degree of signal decrease from flow of blood is less than that from parenchyma). This provides contrast between flow of blood and parenchyma.
However, in the case of the foregoing MR angiography making use of a slice-selective MT pulse, blood flowing into a slice to be imaged suffers from a considerably large amount of MT effects, because a flip angle given to magnetization when the MT pulse is applied is set to a greater amount (for example, 500 to 1000 degrees). This results in MR signals manated from the blood passing the slice to significantly decrease in magnitude. Therefore, contrast between blood and parenchyma is not always fully satisfied under recent needs for higher image resolution.
In addition, in the field of MRI, another clinically significant imaging technique is T2-weighted imaging that emphasizes the T2 relaxation phenomenon.
To perform this imaging requires that the repetition time TR be longer. The entire scanning takes as long as 10 minutes, for example, imposing a greater burden on a patient. To improve this, FSE (Fast Spin Echo) and EPI (Echo Planar Imaging) methods are proposed that produce a plurality of echo signals in response to one excitation pulse.
The EPI method is used to switch a readout gradient between the two polarities to produce field echoes consecutively. This enables single-shot imaging.
The FSE method is characteristic of using a plurality of refocus pulses that are applied after the application of one excitation (shot) pulse and produce multiple echoes. Compared to the EPI method, the FSE method needs a loner scanning (imaging) time, but posses various advantages, such as higher resistance to the non-uniformity of a static magnetic field. Therefore, the FSE method of which number of shots are increased and effective TE (Echo Time) is shortened have been widely used for providing a clinical effectiveness.
On one hand, for depicting cardiac vascular systems, synchronization with the cardiac temporal phase, which is typically represented by an ECG signal, is unavoidable. In addition, field-echo-system pulse sequences whose repetition time TR and/or echo time TE are shortened are used for reduced scanning time in imaging cardiac vascular systems. Particularly, a segmented FFE (Fast FE) can raise a temporal efficiency for scanning, which has been used widely. In the FFE sequences, if the TR or TE is reduced, image contrast is lowered. Thus, to compensate for this lowered contrast, an MT pulse and/or fat-suppression pulse are preferably used. In addition, when considering the fact that three-dimensional imaging takes a long time for scanning, it is, in fact, impossible to force a patient to continue her or his breath hold for such a long scanning time. A continuous imaging is therefore executed in synchronism with an ECG signal. In this ECG-gating imaging, a problem arises in that the heart moves with its respiration. One solution to respiratory motions is selection or correction of data using a navigator echo produced by a navigation pulse incorporated in an imaging pulse sequence.
In general, in cases it is desired to quickly obtain T1-weighted images or PD (proton density)-weighted images, FE-system pulse sequences are superior to others. On the contrary, if T2-weighted images whose sensitivity for lesions is excellent, pulse sequences that are capable of acquiring a plurality of echo signals in response to one excitation time are effective, increasing efficiency in data acquisition. Particularly, for T2-weighted images whose TR or TE is elongated, the EPI or FSE method is preferable and allows total scanning time to be reduced and image artifacts to be suppressed.
However, on account of the fact that conventional T2-weighted images obtained with the EPI or FSE method is based on a multi-slice technique, a signal from blood inflowing into a certain slice to be imaged has already been saturated by the RF excitation for other slices. Hence this T2-weighted imaging provides blood signals of less intensities, being inappropriate for the depiction of flow of blood.
A report has been made that imaging T2-weighted images three-dimensionally makes it possible to depict a vascular system utilizing the characteristic that a region where blood flows slowly is longer in the T2 relaxation time than a parenchyma region. However, even when using this reported technique, images of a sufficient contrast between blood and tissue are still unavailable, independently of the magnitudes of flow of blood. In this three-dimensional imaging, when the number of slices excited per unit time are reduced, there is a tendency that it becomes difficult to reduce signals from muscle or the liver, the signal reduction resulting from MT effects according to the FSE method. Thus the contrast between such tissue and blood is lowered.
As described above, 3D imaging with a continuous breath hold is normally difficult or impossible. In the conventional 3D MR imaging or 3D MR angiography for the abdomen, an intermittent breath hold may have been carried out to reduce artifacts due to respiratory body motions. A respiration-gating technique may have been used for achieving timing to a patient's respiration.
However, since there is no steady way of informing a patient of the timing of the respiration, this respiration gating has not been popularly used in normal clinical fields. An operator needs to instruct a patient to hold the breath at imaging sites each time of the intermittent breath holds. Thus, operative burdens to the operator will be increased. There is a large possibility that respiration timing fails and image quality becomes poor.